Lateral wellbores, or "laterals", offer the potential to drain more oil than would be recovered otherwise. For example, laterals may be used to tap fresh oil by intersecting fractures, penetrating pay discontinuities, and draining up-dip traps. Lateral re-completions can also correct production problems such as water coning, gas coning, and excessive water cuts from hydraulic fractures which extend below the oil-water interface. Moreover, synergistic benefits may result from coupling lateral recompletions with enhanced recovery techniques to solve conformance problems, to contact unswept oil by recompleting injection wells, and to redirect sweep by converting existing well patterns into line-drive configurations. Finally lateral recompletion strategies can take advantage of current production infrastructure, capital resources of existing wellbores, known resources of oil in place, and secondary and tertiary recovery technology.
One major impediment to the widespread use of lateral re-entries is the need to keep the cost of drilling and completing laterals as low as possible. Workover economics in mature fields require substantial cost reductions over the methods most often used for drilling new horizontal wells. Thus, there is a great need for a reliable reduced-cost drilling system that utilizes the equipment and cost structures of workover and repair services.
In addition, to the economic constraints, there are technical limitations. For a curve drilling system to be technically successful it should preferably drill a consistent radius of curvature and drill the curve in the desired direction. This is because it is highly desirable to:
Position the end of the curve within a precise depth interval so the lateral can traverse the pay zone as desired. PA1 Place the lateral in a direction dictated by well spacing, desired sweep pattern, or other geological considerations. PA1 Establish a smooth wellbore to facilitate drilling the lateral and completing the well.
Rotary-steerable drilling systems are one category of curve drilling systems. The downhole components of such systems often include a curve assembly, flexible drill collars, and orientation equipment. The curve assembly is relatively short and incorporates a flexible joint that is pushed to one side of the wellbore to tilt the drill bit. Orientation equipment typically comprises a standard mule-shoe sub for magnetic orientation. This basic system concept has been around for decades; however problems with angle build and directional control have limited its commercial success.
Several tools have been disclosed for drilling a curved borehole. U.S. Pat. Nos. 4,699,224 and 4,739,843 to Burton (and assigned to Amoco Corporation) disclose one basic curve drilling assembly. U.S. Pat. No. 5,213,168 to Warren et. al. (also assigned to Amoco Corporation) describes an alternative and improved curved drilling assembly. Consistent performance in the Warren tool was achieved, in part, by stabilizing the drill bit to continually point along a curved path and designing the bit so that it cuts only in the direction it is pointed. In particular, improved bit stability was achieved by using a "low-friction gauge" technique. (See, for example, U.S. Pat. Nos. 5,010,789 and 5,042,596 to Brett et. al. and assigned to Amoco Corporation). The drill bit cutters are positioned so that they direct a lateral force toward a smooth pad on the side or gauge portion of the drill bit. The pad contacts the borehole wall and transmits a restoring force to the drill bit. This force rotates with the bit and continually pushes one side of the drill bit (i.e., the one that does not have a gauge cutting structure) against the borehole wall. When such a drill bit is used, the curve drilling assembly drills a curved path by continually pointing the drill bit along a line that is tangent to the curved path. The assembly runs smoothly, the hole is uniform in diameter, and the effects of varying lithology are negated. Moreover, the cost to manufacture such an assembly, including the anti-whirl drill bit, is much less than that for a curve drilling assembly that uses a mud motor.
Although the drilling system described in U.S. Pat. No. 5,213,168 drills true, it must be oriented in the desired direction. In particular, many such drilling systems make use of an eccentric deflection sleeve to direct the lower portion of the drillstring and to tilt the drill bit. The orientation of the sleeve determines the azimuth of the curve. Thus, the sleeve must be initially oriented in the target direction and its orientation must be monitored and adjusted (if necessary) as drilling progresses (e.g., because the sleeve may slip and require repositioning.)
U.S. Pat. No. 4,948,925 to Winters, Burton, Warren and Brett (and assigned to Amoco Corporation) describes one apparatus and method for rotationally orienting a borehole engaging means or deflection sleeve. In particular, the sleeve is oriented by turning the drillstring counter-clockwise to have a spring-loaded latch on the mandrel engage a pocket on the sleeve. Further rotation of the drillstring moves the sleeve to the desired orientation. Thereafter, when the drillstring is rotated clockwise for normal drilling, the latch disengages and the sleeve remains in its adjusted position.
U.S. Pat. No. 4,899,833 to Warren and Winters (and assigned to Amoco Corporation) describes one means by which the orientation of a downhole steering assembly is communicated to the drilling engineers at the wellhead. In particular, a downhole valve is used,,to provide a signal at the wellhead to assist in orienting the deflection sleeve. When a reference point on the drillstring is aligned with the maximum eccentricity of the sleeve, the valve reduces the pump pressure by porting fluid above the drill bit. The valve comprises a slotted stationary ring attached to the deflection sleeve and a rotating port in the mandrel that passes through the sleeve. For simplicity of operation, the reference points are aligned and the latch engages at the same time.
Although the above-described drilling systems have many advantages over the prior art and have found commercial success, experience has shown that there is still room for improvement and further development. In particular, improvement is needed in the efficiency and means by which the drill string is oriented in response to operations conducted by drilling engineers at the well head.